1. Field
The present invention relates generally to communication, and more specifically to techniques for determining the reliability of transmit power control (TPC) commands received in a wireless communication system and for combining received TPC commands in soft handover.
2. Background
In a wireless communication system, a terminal (e.g., a cellular phone) communicates with one or more base stations via transmissions on the downlink and uplink. The downlink (i.e., forward link) refers to the communication link from the base stations to the terminal, and the uplink (i.e., reverse link) refers to the communication link from the terminal to the base stations. The downlink and uplink are typically allocated different frequency bands.
In a Code Division Multiple Access (CDMA) system, each base station may receive uplink transmissions from multiple terminals. Since these uplink transmissions occur over a shared frequency band, the transmission from each terminal acts as interference to the transmissions from other terminals. For each terminal, the interference due to other terminals degrades this terminal's received signal quality. (For simplicity, signal quality is quantified by a signal-to-interference ratio (SIR) in the following description.) Each terminal needs to maintain a particular SIR in order to achieve the desired level of performance, which may be quantified by a particular frame error rate (FER), packet error rate (PER), block error rate (BLER), or bit error rate (BER).
To maximize uplink capacity, an uplink power control mechanism is typically used to control the transmit power of each terminal. Conventionally, this power control mechanism includes an “inner” loop and an “outer” loop. The inner loop adjusts the transmit power of the terminal such that its received SIR, as measured at the base station, is maintained at a target SIR. This target SIR is often referred to as the “setpoint”. The outer loop adjusts the target SIR to maintain the desired level of performance, which may be, for example, 1% BLER. The uplink power control mechanism thus attempts to reduce power consumption and interference while maintaining the desired link performance for the terminal.
Many CDMA systems support soft handover (i.e., soft handoff) on the uplink. Soft handover on the uplink refers to the reception of an uplink transmission from a terminal by multiple base stations. Soft handover provides diversity against deleterious path effects, such as fading and multipaths, since multiple base stations at different locations are used to receive the uplink transmission. Soft handover may provide improved reliability for the uplink transmission from the terminal.
While a terminal is in soft handover on the uplink with multiple base stations, an inner loop is typically maintained by each of these base stations to adjust the terminal's transmit power. Conventionally, each base station determines the received SIR for the uplink transmission, generates TPC commands based on the received SIR and the target SIR, and transmits the TPC commands to the terminal. Each TPC command is either (1) an UP command to direct the terminal to increase its transmit power or (2) a DOWN command to direct the terminal to decrease its transmit power. Each base station independently generates TPC commands for the terminal based on the received SIR measured at that base station for the uplink transmission from the terminal.
While in soft handover, the terminal receives TPC commands from multiple base stations and determines whether each received TPC command is an UP command or a DOWN command. The terminal then conventionally applies the “OR-of-the-DOWN” rule for the TPC commands received from multiple base stations. For each power control period, this rule requires the terminal to decrease its transmit power if any one of the received TPC commands is determined to be a DOWN command.
The OR-of-the-DOWN rule works well if the terminal can reliably receive the TPC commands from all base stations. If the TPC commands from any base station are unreliably received, then the transmit power of the terminal will be biased downward or low. This is because UP commands sent by an unreliably received base station may be erroneously detected as DOWN commands by the terminal. This would then cause the terminal to decrease its transmit power regardless of the TPC commands received from more reliable base stations. The downward bias in transmit power can result in degraded performance (e.g., a higher BLER) for the terminal. In extreme instances, the downward bias can lead to a loss of the terminal's signal at the base stations.
Since the downward bias and higher BLER are undesirable, a requirement is often imposed such that only TPC commands received with sufficient reliability are combined by the OR-of-the-DOWN rule. Conventionally, the reliability of the received TPC commands for each base station is determined based on a pilot received from that base station. Each base station transmits its pilot at a fixed power level. If the received pilot strength (i.e., received pilot power) for a base station exceeds a pilot threshold, then that base station is considered to be received reliably and its TPC commands are combined by the OR-of-the-DOWN rule.
The use of received pilot strength to determine the reliability of received TPC commands is sub-optimal for several reasons. First, different transmit power levels may be used for the pilot and the TPC commands. The pilot is transmitted at a fixed power level while the TPC commands are transmitted at a power level determined by a downlink power control mechanism. If the TPC commands are transmitted at a low power level relative to the power level of the pilot, then the TPC commands may be unreliably received even if the pilot is strongly received. Second, if power balancing is not applied while in soft handover, then the ratio of TPC command power to pilot power may differ widely for different base stations. In this case, even if the received pilot strengths are the same for all base stations, the TPC commands will be received at different powers for different base stations. Thus, in certain circumstances, the reliability of the received TPC commands cannot be accurately determined based on the received pilot strength.
There is therefore a need in the art for techniques to more accurately determine the reliability of received TPC commands and to combine these received TPC commands in soft handover.